An electroluminescent (EL) device is a self-light-emitting device which has advantages in that it provides a wider viewing angle, a greater contrast ratio, and a faster response time.
An organic EL device was first developed by Eastman Kodak, by using small aromatic diamine molecules and aluminum complexes as materials to form a light-emitting layer [Appl. Phys. Lett. 51, 913, 1987].
Generally, the organic EL device has a structure comprising an anode, a cathode, and an organic layer disposed between the anode and the cathode. The organic layer of the organic EL device comprises a hole injection layer, a hole transport layer, an electron blocking layer, a light-emitting layer, a hole blocking layer, an electron transport layer, an electron injection layer, etc. When a voltage is applied to the organic EL device, holes and electrons are injected from an anode and a cathode, respectively, to the light-emitting layer. Excitons having high energy are formed by recombinations between the holes and the electrons. The energy of excitons puts the light-emitting organic compound in an excited state, and the decay of the excited state results in a relaxation of the energy level into a ground state, accompanied by light-emission. Therefore, the most important factor determining luminous efficiency in the organic EL device is light-emitting materials.
Depending on its function, the light-emitting materials can be classified as a host material and a dopant material. Generally, devices showing good electroluminescent characteristics comprise a light-emitting layer in which a dopant is doped into a host. The dopant/host system is provided to enhance luminous efficiency through energy transfer from a host to a dopant. In the dopant/host material system, the dopant and host materials highly affect efficiencies and lifespan of the device.
Iridium(III) complexes have been widely known as phosphorescent materials, including bis(2-(2′-benzothienyl)-pyridinato-N,C-3′)iridium(acetylacetonate) ((acac)Ir(btp)2), tris(2-phenylpyridine)iridium (Ir(ppy)3) and bis(4,6-difluorophenylpyridinato-N,C2)picolinate iridium (Firpic) as red-, green-, and blue-emitting materials, respectively.
However, considering EL characteristic requirements for a middle or large-sized panel of OLED, Iridium(III) complex-based dopant compounds showing better characteristics, i.e., long lifespan, high efficiency, and high color purity must be urgently developed.
Particularly, for a full-color display, pixels which show particular colors called as a “saturated” color are required, and especially, saturated red, green, and blue pixels are required, which can be measured by a CIE coordinate known in this field. In order to display more various colors, each of red, green, and blue should have high color purity. In the case of red, color purity becomes higher, as it approaches a deep-red coordinate around 0.680 of x-axis of CIE. A dopant compound which meets such requirements is desired.
Korean Patent No. 0662381, Korean Patent Application Laying-Open No. 2011-0086021, WO 2008/109824, and WO 2010/033550 disclose iridium complexes having 2-phenylquinoline-based ligand, as a dopant compound of an organic electroluminescent device. However, said prior arts fail to specifically disclose an iridium complex having a ligand in which quinoline of 2-phenylquinoline is substituted with alkyl at 4-position and is substituted with phenyl at 6-position.